The touch screen has many advantages such as ruggedness, fast response, space saving, and easy communication. Compared with keyboard input, the touch screen technology is simpler, more intuitive, faster, and has rich and colorful performance capabilities. When designing the touch screen, the most important problem is to accurately locate the coordinate position of the touch point. This paper describes in detail the practical scheme of designing the four-wire resistive touch screen using the industrial grade chip LPC2478 and ADS7843.
1 hardware design
1.1 Hardware Selection The LPC2478 chip integrates an LCD interface. Its working range is -40 to +80 °C. Its wide temperature is especially suitable for industrial applications. The ADS7843 chip is a chip designed for touch sampling with 12-bit programmable accuracy. The external reference voltage range is from 1 V to Vcc, and Vcc is up to 5 V. High speed and low power consumption make the ADS7843 ideal for handheld devices with resistive touch screens.
1.2 Hardware Circuit The author designed the hardware interface circuit of ADS7843 (as shown in Figure 1). The circuit uses the GPIO pin of the LPC2478 to simulate the SPI timing. Connect DCLK, CS, DIN, BUS-Y, and DOUT to the five GPIO pins of the LPC2478. Will ADS7843 
The pin is connected to the LPC2478 interrupt 1 mode. The four-wire resistive touch screen is connected to the X+, Y+, X-, Y- pins of the ADS7843.
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1.3 Acquisition Mode The ADS7843 has two reference voltage modes: single-ended mode and differential mode. In single-ended mode, the reference input voltage is chosen to be Vcc and GND. Since the internal switch resistor voltage drop affects the conversion result, the low-resistance switch inside the converter has a certain influence on the conversion accuracy. The differential mode reference input provides reference power and ground from unselected input channels Y+, Y-, X+, X-. Regardless of how the internal switch resistance changes, the conversion result is always proportional to the resistance of the touch screen, overcoming the effect of the internal switch resistance, but when the conversion frequency is high, the power consumption is increased, and the low power design needs to be considered. The author uses a differential method in the program based on the reason of sampling accuracy.
ADS7843 pin It is a PIN interrupt pin that generates a low level when the display is touched, triggers the LPC2478 interrupt, and uses a falling edge trigger. The process of one touch of the touch screen captures the level change process of Figure 2. It is not very suitable for collecting level changes by using interrupt mode, and it is unable to capture the sliding process on the touch screen. Therefore, the author uses the cyclic acquisition method.
Cyclic acquisition mode: mainly through the LPC2478 timing acquisition of ADS7843, the timer interval in the author's program is set to 50 ms. This method is not suitable for the slower MCS51 microcontroller because it increases the CPU load, but it is more suitable for the LPC2478 processor. The data collected each time is compared with the last data to determine whether it is new data.
The research mainly uses LPC2478 and ADS7843 for display and external input, no other large calculations, and no real-time control of the equipment. Therefore, the advantages and disadvantages of comparing the interrupt mode and the loop mode are mainly to respond to some sliding actions on the screen, and finally adopt the loop acquisition mode.
1.4 ADS7843 acquisition process uses 12-bit differential mode acquisition mode, the control words in the program are 0xg0 (x coordinates) and 0xD0 (y coordinates). According to the timing diagram of ADS7843, the author collects the program's workflow: after the control word is sent after 8 clock cycles, it is read 12 times continuously on the falling edge of DCLK, thus reading the AD sample value on the touch screen. Due to the cyclic acquisition method adopted by the author, the collected data is not necessarily the data generated by the touch of the person on the screen. In the author's system, the LPC2478 LCD screen uses the Sharp LQ043T3DX02 LCD screen, and the data is collected if the x-axis data is 4 095 (the y-axis data is mostly collected to O, but some times may not be 0). Note that when the data is collected, no one touches the screen, and the data can be directly discarded.
In practice, it is not only concerned with the ADS7843's current A/D conversion value of the touch point voltage value, but also the relationship between the touch point and the LCD coordinates. It can be converted by the following conversion formula: 
Where: x, y are the coordinates in the LCD coordinates; xAD, yAD are the AD values ​​collected by the ADS7843; Tchscr_XMax, Tchscr_XMin, Tchscr_YMax, and TchScr_YMin are the ranges of the x, y coordinates returned by the touch screen.
2 Conclusion The practical scheme of designing a four-wire resistive touch screen using the chip LPC2478 and ADS7843 described in this paper achieves specific functions. The practical application shows that the touch screen designed by LPC2478 has strong reliability and environmental adaptability.
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